In order to ensure safety of important equipment such as nuclear reactors, aircrafts, railways and the like and to ensure soundness of manufactured materials and joined materials, safety control is executed in which a crack and an incomplete joined surface which cause destruction are detected by reflection or scattering of ultrasonic waves, the size thereof is accurately evaluated and the equipment is replaced if danger is present. However, in a fatigue crack having large crack closure stress that closes a crack surface due to various causes or a closed crack in which an oxidized film is formed on the crack surface caused by stress corrosion crack, reflection/scattering of ultrasonic waves is small, and a measurement error of a length and a depth of a crack is large, which is a problem.
A process of obtaining an image of a crack by adding a signal obtained by giving a different delay according to a position of an element to scattered waves obtained by the reception array element is called a phased-array method, which is known in the field of non-destruction inspection (See Non-Patent Document 1, for example). However, even with an image by the phased-array method, accurate measurement of a closed crack has been difficult.
Under the above circumstances, a quantitative evaluation method of a closed crack using subharmonic waves generated in a closed crack by radiating ultrasonic waves with a large amplitude and a quantitative evaluating device of a closed crack are proposed (See Patent Document 1, for example). With this evaluation method and evaluating device, by applying the phased array method to the subharmonic waves, a closed crack can be imaged. This method is named SPACE (Subharmonic Phased Array for Crack Evaluation) (See Non-Patent Document 2, for example).
FIG. 14 is a principle diagram of the SPACE. In the SPACE, a transmission-side probe (transmitter) using a LiNbO3 single crystal oscillator with excellent pressure resistance that can generate ultrasonic waves with a large amplitude, an array receiver for imaging, and a frequency pass filter (digital filter) are used as an element technology. By radiating large-amplitude ultrasonic waves (frequency f) from the transmission-side probe, in addition to linear scattering of fundamental waves (frequency f) at an open crack, subharmonic waves (frequency f/2) are generated since the closed crack surface is opened by tensile stress of the large-amplitude ultrasonic waves at the closed crack and opened/closed oscillated. The waves are received by the array receiver, and components are separated by the digital filter so that a fundamental wave image and a subharmonic wave image can be observed. If a crack distal end is closed, a crack depth might be underestimated with the fundamental wave image, but it can be accurately measured with the subharmonic wave image.